The invention relates to apparatus and methods for loading quartz boats of semiconductor wafers into diffusion furnaces for processing at elevated temperatures, without generating excessive numbers of defect-causing particulates, and relates more particularly to cantilever apparatus for moving diffusion boats and wafers supported thereby into diffusion furnaces without quartz-to-quartz abrasion or contact.
A variety of semiconductor processing operations are commonly performed in diffusion furnaces, which in a modern semiconductor wafer fabrication facility frequently include two "stacks" of diffusion furnaces placed side by side. Each stack typically includes four horizontal quartz "diffusion tubes", each approximately eight feet long, positioned each above the other in a "diffusion furnace". The two stacks are positioned back to back, each being accessible from an opposite side. At one end of each stack is a "source cabinet" in which connections to controlled sources of various reactant gases can be made to the "pigtail" end of each diffusion tube. The opposite "mouth" end of each diffusion tube extends into a "scavenge box" into which used reactant gases are exhausted and conducted to a "scrubber" that performs the function of burning off certain components of the exhausted gases. A "load station" for each diffusion tube is connected to the loaded end of each diffusion furnace.
Those skilled in the art will realize that the foregoing arrangement of back to back stacks is necessary to minimize the amount of floor space required because it is known that an ultra-pure environment must be maintained in a modern wafer fabrication facility to avoid, to the greatest extent possible, the existence of particulates, even those in the range from 0.5 microns to 4 or 5 microns in diameter, in the ambient air. This is because it is well known that particles of this size can cause defects in the integrated circuits being manufactured in the wafers. The resulting decrease in wafer yield (and hence the increase in fabrication cost per integrated circuit) increases with the density of such particulates in the wafer fabrication environment. As state of the art of integrated circuits proceeds toward minimum line widths, and line spacings are reduced toward one micron, the minimum size of a typical particle that will cause a catastrophic defect in an integrated circuit becomes smaller and smaller. Tremendous amounts of capital have been invested by the semiconductor industry over the past decade or so to improve purity of the air and environment which is required for high yield wafer processing. Floor space in such a modern wafer fabrication facility is extremely expensive.
The various wafer processing operations mentioned above typically include semiconductor diffusion operations at high temperatures of over 1,000.degree. C., and also somewhat lower temperature processes, including thermal oxidation and LPCVD (low pressure chemical vapor deposition) processes such as deposition of silicon nitride or polycrystalline silicon on semiconductor wafers.
In order to perform the foregoing processing operations, it is necessary to load quartz diffusion boats, each holding typically 50 to 75 four inch or five inch partially processed semiconductor wafers, into the open end of the quartz diffusion tube of a diffusion furnace. Often this has been accomplished using "paddles" which are quartz platforms with quartz wheels that roll along the lower inner surface of the horizontal diffusion tube to convey the wafers into a "hot zone" of the diffusion furnace, whereat the temperature of the wafers is elevated and stabilized at the desired level for the desired oxidation, diffusion, or a chemical vapor deposition process. Quartz-to-quartz abrasion occurs in such loading systems, generating quartz particles that are commonly referred to as "quartz dust" and are capable of causing defects in the integrated circuits if they settle on the surface of the semiconductor wafers. Not only do such defects reduce the yield by causing some of the integrated circuits to fail function tests, but they also sometimes produce latent defects which allow the integrated circuits to pass functional tests and hence, are sold, but lower the longer term reliability of these integrated circuits.
For LPCVD processes, the silicon nitride or polycrystalline silicon layers which are deposited upon the exposed surfaces of the semiconductor wafers are also deposited on the inner surface of the diffusion tubes. The wheels of the paddle roll on the deposited material on the inner surface of the diffusion tube, causing pieces of the deposited material to break off, thereby generating large numbers of defect-causing particles, some of which settle on and adhere to the semiconductor wafer surfaces. Furthermore, both silicon nitride and polycrystalline silicon layers on quartz have greatly different coefficients of thermal expansion than quartz, causing great stresses at the quartz interface as the diffusion tube temperature is decreased. These stresses can cause breaking off of defect-producing silicon nitride or polycrystalline silicon particles which may settle on and adhere to a wafer surface. Furthermore, the interface stresses also cause surface fractures in quartz, which fractures can spread in the quartz, causing premature breakage.
The damage that can be caused by particles produced by the foregoing wafer loading and unloading processes are severe enough that "cantilever" loading systems have been developed and marketed by several manufacturers, wherein two parallel quartz covered cantilevered metal rods are supported at one end from a carriage or "driver" mechanism and are movable into and out of a diffusion tube while supporting one or two boat loads of wafers. The two quartz rods extend from a "door" plate which forms a seal with the flanges of the mouth of the diffusion tube, preventing escape of reactant gases. These cantilever devices, when operating properly, substantially eliminate quartz-to-quartz abrasion during the wafer loading and unloading operations, resulting in extremely low densities of defect-producing particles within the diffusion tube. However, this advantage has not been attained without introducing other problems that have not yet been solved, nor has the use of such cantilever devices solved some other long-standing problems that decrease yields and increase costs in the wafer fabrication art.
As to problems particularly associated with the above-mentioned prior art cantilever systems, those systems are less than totally satisfactory at the high temperatures that are required for semiconductor diffusion operations because the cantilever rods tend to sag or droop at such high temperatures. Since the length of the quartz rods of such a device is approximately five feet and the weight of each of the wafer-loaded boats is approximately four or five pounds, the maximum number of such loaded boats that can be used on the prior cantilever devices is usually two. This represents a considerable reduction in the number of wafers that can be carried by the above-mentioned paddle loading systems, which typically can carry four or more boat loads of as many as 75 four or five inch wafers. Therefore, the use of the prior cantilever loading devices reduces the throughput rate of a diffusion furnace, and the cost of this reduction must be weighed against the expected increase in yield resulting from the lower density of defect-causing particules generated within the diffusion tube by the devices as opposed to conventional loading and unloading processes using the above "paddles".
The aforementioned "sag" also dictates the processing of somewhat smaller wafer sizes in a given size diffusion tube to allow for wafer-to-diffusion tube tolerances that must be allowed because of the sag.
The inherent flexibility in such cantilevered rod systems sometimes allows physical oscillation to occur in the system during operation of the carriage transport mechanism. This phenomenon further contributes to the tolerance problem and therefore further reduces the maximum wafer size that can be processed in the system.
Even with only two boat loads of wafers supported on its free end, the forces exerted on the prior art cantilever loading device rods are far too great for solid quartz rods to support, so is has been necessary to use hollow quartz rods inside of which much stronger "center rods" of alumina, graphite, or silicon carbide are inserted. Typically, the rear ends of the center rods are clamped by means of a clamping mechanism to a carriage that rides on a linear bearing, such as a Thompson bearing. The portions of such center rods that extend through the "door plate" into the diffusion tube are covered by the hollow quartz rods on which the wafer-loaded diffusion boats rest. Unfortunately, it is not feasible to obtain truly impurity-free alumina graphite or silicon carbide center rods. The rods actually used are believed to contain fast-diffusing contaminants, such as heavy metals and sodium, which have deleterious effects on certain critical semiconductor parameters, such as surface-state charge Q.sub.SS of the wafers, causing reduced wafer yields.
One of the most severe problems with the state of the art cantilever systems is that when the wafers supported thereby are withdrawn from the furnace, the wafers too rapidly encounter ambient atmospheric oxygen as the wafers are moved out of the diffusion tube into the loading station. If this happens before the wafers have had a chance to cool to a low enough temperature (typically about 600.degree. C.), the oxygen will cause unacceptable shifts in Q.sub.SS, unless vast quantities of purging gas (typically nitrogen) are used. Usually, if a conventional paddle system is used, an extension tube sometimes referred to as a "white elephant" is attached to the open mouth of the diffusion tube, and the paddle and wafers thereon are withdrawn from the hot zone of the diffusion tube into the "white elephant" while the purging gas continues to flow, preventing exposure of the wafers to atmospheric oxygen until temperature of the wafers falls below roughly 600.degree. C. Unacceptable Q.sub.SS shifts are avoided without use of excessive amounts of purging gas.
The prior art cantilever loading systems, however, require thousands of times more nitrogen gas during purging than the paddle type loading/unloading systems, and also require much slower withdrawal rates. The nitrogen gas is quite expensive. The slow withdrawal rates add to the length of time required for the process, and consequently, reduce the throughput rate of the diffusion stations; yet the slow withdrawal is necessary to avoid both Q.sub.SS shifts and unacceptable wafer warpage, the latter of which may cause subsequent masking and photoresist problems and may also cause slippage in the semiconductor lattice structure. Such slippage can propagate through the wafer during subsequent high temperature processing steps and generate semiconductor junction defects and thus also cause circuit inoperability.
Another severe shortcoming of the prior cantilever loading systems is that the alumina center rods mentioned above have relatively large area cross sections and present very high thermal mass beneath the wafers. This situation results in non-uniform flow of the reactant gases (which is known to be undesirable) and more importantly, causes significant gradients in the temperature inside the diffusion tube across the diameter thereof. This results in non-uniformity of the process being carried out, whether it be a diffusion process, chemical vapor deposition process, or oxidation process. For example, in thermal oxidation processes, there is typically a variation of 50 angstroms per thousand across the wafers from top to bottom. The above mentioned non-uniformity is undesirable and can cause yield-reducing variations in circuit performance from top to bottom of wafer.
Another problem with the prior cantilever systems is that the wafers are withdrawn from the diffusion tube from the ultra-pure, low defect-causing particle density environment within the diffusion tube into the loading station, which ordinarily is in a non-laminar air flow environment having a considerable density of defect-causing particulates which to some extent negates the desirable low particulate density achieved within the diffusion tube. Due to the structure of typical loading stations and the need to stack them back to back, modifications to provide laminar air flow and the resulting desired low particulate density in the loading station are usually prohibitively costly.
Another problem of prior art cantilever loading systems that has been alluded to above is the reduced number of wafers per run (typically 100 wafers) that can be accomplished with state-of-the-art cantilever loading systems compared to the number of wafers (typically several hundred) for prior paddle systems.
Another problem of prior art cantilever loading systems is that when the hollow quartz tubes through which the alumina rods extend are initially heated to a high temperature, the quartz material sags, and later when the temperature of the rods and quartz is reduced (during a subsequent withdrawal step) internal stresses are generated in the quartz. This stress adds to stresses produced later due to the weight of one or two boat loads of wafers that are placed on the quartz rods. Occasionally, the prior cantilever loading systems fail due to breakage of the center rods or quartz, causing damage to or breakage of the wafers supported thereon. This can be extremely costly, due to the high value of the wafers themselves.
Another very severe shortcoming of the prior cantilever systems is that they require a large amount of labor and "down time" of the diffusion furnace to replace them. The prior cantilever rods need to be replaced fairly frequently, due to build up of contaminants on them or breakage or fracture of the quartz rods. Typically, three to four hours are needed to change the quartz rods, due to the need to "ramp down" (decrease) the temperature of the diffusion tube to allow working in the vicinity, and also due to the need to achieve extremely precise alignment and clamping of the alumina rods to the carriage mechanism so that stresses on the hollow quartz rods and quartz "bridges" interconnecting the rods are avoided (as breakage otherwise would be likely to occur).
Another problem with the prior cantilever systems is that due to the large cross sectional area of the rods that support the wafer-loaded quartz diffusion boats, the maximum size of wafers that can be used in a diffusion furnace of a particular diameter is not as great as would otherwise be the case. Since there is a present trend in the industry to increase the size of wafers processed from five inches to six inches, it will be necessary, if prior cantilever devices are to be used for wafer fabricators, to use larger diameter diffusion tubes that are much more expensive, and which in some cases can only be stacked three deep rather than four deep at each diffusion station. This will increase the amount of expensive floor space needed in the wafer fabrication area.
Another problem with some of the state of the art cantilever diffusion systems is that the carriage and the alumina rods conduct too much heat to the carriage mechanism. This has caused vaporization of grease in the bearing mechanism and when the wafers are withdrawn, some of this vaporized grease has been redeposited in the form of carbon films on the semiconductor wafer surface. This can cause reductions in wafer yield.
There are several long-standing problems that have been common to all prior loading systems and diffusion systems. One has been the need to frequently clean diffusion tubes, which become contaminated every 10 to 15 wafer processing operations or runs. In order to clean quartz diffusion tubes, it is necessary to ramp the temperature gradually down to a temperature at which the tube can be either removed for cleaning or cleaned in situ. The ramping rate is typically only 4.degree. C. per minute, so the ramping down process can take four to ten hours, depending on its initial temperature. After the diffusion tube has been cleaned, which requires a considerable amount of labor and large amounts of expensive ultrapure chemicals (which then must be disposed of at significant expense), the diffusion tube must then be "ramped up" to the proper operating temperature. Again, this can take many hours. The result of the need to frequently clean diffusion tubes is that for as much as one-third to one-half of its total lifetime, the diffusion tube is not being used for wafer processing. Furthermore, in diffusion tubes in which LPCVD processes are carried out, the above mentioned damage in the form of surface fractures to the quartz (due to the above mentioned large differences in coefficients of thermal expansion of silicon nitride and polycrystalline silicon compared to quartz) shortens the lives of expensive quartz components.
Quartz "liners" have been used in the past. These are cylindrical tubes that are used to line the diffusion tubes. They can be installed more easily than the diffusion tubes, and can be removed more easily for cleaning (after they become contaminated by 10 to 15 runs) than the diffusion tubes. However, these liners are generally subject to all of the shortcomings mentioned above, and also to the one mentioned next.
Another long standing problem in the LPCVD silicon nitride deposition process is sometimes referred to as "streaking". This occurs when wafers are withdrawn from a silicon nitride deposition process. It is thought that ammonium chloride that sublimates on the internal surfaces of the colder portions of the diffusion tube (or liner) later vaporizes when the hot wafers are withdrawn past the sublimated ammonium chloride at the mouth of the diffusion tube, and then is redeposited in the form of streaks or haze on the surface of the wafer as it is withdrawn. Although it is not known precisely what effect this has on wafer yield, it is suspected that it probably decreases the effectiveness of subsequent masking and photoresist operations and decreases overall yield.
One technique that has been used in the past, and is believed to be still in use in experimental semiconductor processing is the use of sealed quartz ampules in which wafers are sealed with reactant gases before the ampules are pushed into a diffusion furnace. This technique can produce a very pure, particulate-free atmosphere within the ampules during the diffusion process by avoiding quartz-to-quartz abrasion that generates defect-causing particulates. However, the ampules must be broken and thus destroyed after removal and cooling of the ampules to recover the wafers. Furthermore, particulates ordinarily would be generated when the ampule is broken. This would necessitate careful subsequent cleaning of the wafers to avoid the resulting particles from causing defects. This approach clearly is not presently suitable for high volume, high yield wafer production processes.